DC brushless motor fans come in a variety of shapes and sizes. There are a number of fan types including radial, axial, or blower designs, and the fan type determines the airflow direction with respect to the fan body. Airflow rate is one of the most important specifications for a fan as this quantifies the fans ability to circulate air. This can vary in volume depending on fan blade design, fan construction, and speed of rotation. Most of these properties are set at the fan design stage, leaving only the speed of rotation available to the user to determine the airflow rate and hence the amount of cooling needed to maintain a safe operating environment. In order to control this fan speed parameter we need to look at the underlying controlling circuit in a fan.
A basic DC brushless motor consists of a number of key components and will be well known to those in the art. The motor includes a fixed part, the stator having a fan housing and an electronic assembly. The motor also includes moving parts including a rotor comprising a fan blade and a circular permanent magnetic strip attached to it. The electronic assembly contains a number of coils that can be electronically energised in sequence as the fan rotates. The energising sequence is synchronised to the fan rotation by a Hall effect sensor. This sensor is placed close to the circular permanent magnetic strip on the rotor and as the fan rotates the sensor detects the magnetic field of the passing magnet. The sensor output changes state every time a magnetic pole change is detected, i.e., going from a North Pole to a South Pole and visa versa. The permanent magnetic strip is made up of a number of magnetic pole pairs and thus it is possible to use the detection of the number of state changes of the sensor output to determine one full rotation of the fan.
The basic DC brushless fan has a 2-wire connection, one connects to a DC power source, and the other is the ground return. Becoming more common now is a third wire known as the ‘tachometer output’ or ‘TACH’. This is a feedback signal from the fan representing the speed the fan is rotating at; in effect this is the Hall effect sensor output signal. An example of a TACH signal is shown in FIG. 1. As will be seen the signal is a square wave with a number of rising and falling edges separated by respective “high” and “low” levels. Each edge corresponds to a passage of a respective pole of the permanent magnet past the Hall sensor, so in effect a complete rotation of a fan having a four pole magnet will include two rising edges and two falling edges; a six pole magnet three rising edges and three falling edges and an eight pole magnet four rising edges and four falling edges. The fan construction determines the number of poles of the fan; this is the number of pole pairs that are passed in one complete rotation of the fan blade. To understand how this signal is generated we need to look at the basic fan drive circuit of a DC brushless motor, an example of which is given in FIG. 2.
The three leads from the fan are labelled:
The power lead, Vdd,
The ground lead, GND,
The Tachometer signal, TACH.
When Vdd is powered, the hall-effect sensor will initially be at a steady state, say GND. This is applied to the gate of transistor T1, keeping it in the off state, so coil 1 has no current flowing through it and therefore no voltage dropped across it. This in turn means the gate of T2 is pulled high, turning it on and therefore energizing coil 2. Energizing coil 2 has the effect of rotating the fan blade and once the hall-effect sensor detects the next pole change, its output changes state to high and the transistor states change, T2 turns off and T1 turns on, thus energizing coil 1, and causing the fan blade to rotate further until the next pole change is detected by the hall effect and the process repeats again.
Although a fan could be designed to operate either in a fully on or a full off position, it is more normal for the fan to operated at a number of different requirements depending on the conditions where the fan is operating. As such it is necessary to enable a control of the fan speed. There are several existing control schemes for controlling fan speed, including linear and pulse width modulation (PWM) control.
Linear control is also sometimes referred to as voltage control and relies on the principal that changing the voltage drive level to the fan will proportionally change the fan speed. Some fans will operate in this way, but not all. Typically, fans with internal electronic driver integrated circuits (ICs) require some minimum DC voltage to operate and therefore will stall below this level, sometimes this can be as high as 50% of the maximum allowed DC level.
In order to generate the variable DC level an interface circuit such as that shown in FIG. 3 is required. A digital-to-analogue converter supplies the variable voltage to this interface circuit and it then acts as a buffer to supply the current load demanded by the fan. At slower speeds the reduced voltage level across the fan means there is a larger voltage drop across the pass transistor Q1, therefore a lot of power is lost especially if the fan requires large current drive, and this in turn acts as a heat generator which stacks against the reason for needing a fan in the first place to cool the system. However, despite the drawbacks this control method has good acoustic performance as the reduced DC level helps to minimise the acoustic noise generated by the fan coil switching as it rotates. Therefore, although linear control is advantageous in that it is a proven technique endorsed by many fan vendors, has known reliable operation, there is extensive historical data available, has good acoustic performance and is easy to measure using the TACH signal it also suffers from a number of disadvantages. These include the fact that some fans do not operate at the lower voltage levels and this can reduce the range of speed of the fan. It is also difficult to determine the fan start and stall voltages and this requires calibration or characterization of every fan. Furthermore, such operation effects large power dissipation in power transistors (>5A fans) due to the voltage drop across transistor. This may require multiple external components such as for example amplifier/buffers, power transistors, gain setting resistors etc.
An alternative control technique is provided by pulse width modulation (PWM). PWM control is probably the simplest fan speed control technique available. The fan speed is changed by varying the Duty Cycle of a square-wave drive signal applied to the fan interface circuit, as shown in FIG. 4. The scheme is quite effective for most fan types but can present difficulties. The primary drawback is the fact that the PWM drive signal is not synchronized to the internal fan electronics, therefore the fan is being pulsed on and off regardless of where the fan is in its rotation cycle. The other drawback is the loss of the speed information. The PWM drive is asynchronous to the tachometer (TACH) output and there is therefore no way of knowing when successive TACH output changes occur. One way around this is to periodically keep the drive on for sufficient time to gather this speed information, thereby stretching the drive on-time.
FIG. 5 shows the PWM signal being held on for 1 TACH period. This stretching out of the drive on time can be for 1, 2 or more TACH periods depending on the accuracy required for the speed measurement. In order to maximize the speed accuracy, this stretching should last long enough to allow the fan complete one full rotation. However, stretching the drive on for this length will cause a very noticeable speeding up of the fan, particularly at lower speeds. This stretching can also cause a ‘hunting’ noise effect where the fan appears to speed up momentarily while the speed information is gathered. Therefore, although there are a number of advantages associated with this technique including the fact that it is a relatively simple low side driver fan drive interface, is a low cost solution and has relatively good speed control over a range of values, these have to be compared with the associated disadvantages such as the fact that the TACH signal is destroyed by drive switching, a 30 Hz to 100 Hz PWM frequency can cause audible ticks and buzzes, and the pulse stretching technique to measure fan speed can be audible at lower speeds, and causes speed variation.
Although hereinbefore described with reference to two and three wire fan systems it is also known to have a four wire system, the fourth wire providing an external feed or sync signal to the fan. U.S. Pat. No. 6,381,406 assigned to the Hewlett Packard Corporation describes a method and apparatus for controlling the speed of such a four wire voltage-controlled fan by locking the pulse-width modulated speed control voltage to a tachometer signal of the fan. By triggering the off time of the PWM pulse to the detection of the tachometer signal and ensuring the off time is less than one tachometer period, no phase and frequency information is lost. The synchronization is achieved by providing the fan controller with a system-generated speed signal SYNC and a tachometer signal TACH from the fan. When a voltage is applied to the fan, the fan rotor begins to spin and generates a tachometer signal TACH one or more times per full revolution of the rotor, each TACH signal having a fixed TACH period. The controller generates a pulse-width modulated signal PWM OUT which is used to turn the fan motor on and off. The controller adjusts the width of the PWM OUT pulses to the fan such that the fan's speed will either decrease or increase until the TACH signal matches the frequency and phase of the control signal SYNC. In order to allow the controller to properly operate using low-frequency PWM signals, the controller synchronizes the off time PWM OUT signal with the detection of the TACH signal and guarantees that the off time is always less than one TACH period. This ensures that the power to the fan is always turned on by the time the TACH signal arrives, and is therefore detected by the controller. Accordingly, accurate TACH data is available for calculation of the next “off” period of the power to the fan, and pulse width modulation can be accomplished at a TACH frequency of less than 200 Hz without losing any TACH phase or frequency information. However, as the synchronization requires a monitoring and correlation between the speed signal and the TACH signal, it can only be implemented in a four-wire system. Such systems, although known, are not the most prevalent of available fans and it is therefore desirable to provide a fan controller that enables a synchronization of the PWM drive signal yet does not require an externally generated sync signal to achieve this synchronization.